1. Field of the Invention
The present invention relates to a solar cell, and more particularly to an electrode of a wafer type solar cell and a fabricating method thereof.
2. Description of Related Art
Solar energy is a renewable energy and causes no pollution. To counter problems including pollution and shortage of fossil fuels, the solar energy has always been the most prominent energy. Here, solar cells have become a very important research topic at present because the solar cells can directly convert the solar energy into electrical energy.
The most basic structure of a typical solar cell can be divided into four main parts: a substrate, a P-N junction, an antireflective layer, and two metal electrodes. Generally, the electrodes in the solar cell module are respectively disposed on a non-irradiated surface and an irradiated surface for connecting external circuits. The non-irradiated surface is regarded as a backside, and the irradiated surface is regarded as a front side. The electrode disposed on the irradiated surface (the front side electrode) is designed for improving efficiency of the solar cell. Currently, a method of fabricating the front side electrode frequently adopted in the industry refers to implementation of a screen printing process with use of metal paste containing metal powder, glass frit, and organic carriers and a sintering process for curing the metal paste. However, the existing method of fabricating the front side electrode has following drawbacks.
The silver paste penetrates through an antireflective layer (silicon nitride) to adhere the metal electrodes to a silicon substrate tightly by the glass frit in the silver paste. During said penetration of the glass frit, since the sintering process is frequently performed at an excessively high temperature or for an overly long time, the metal electrodes overreact with the silicon substrate, and the electrodes then penetrate a P-N junction (having a depth approximating to 0.5 micrometers) on a surface of the solar cell, thereby reducing or even deteriorating the efficiency of the solar cell.
A diameter of the glass frit ranges from 10 micrometers to tens of micrometers, thus confining the minimum line width of conductive grid in the electrodes. In most cases, the line width of the conductive grid is greater than 70 micrometers. Hence, the existing method of fabricating the front side electrode cannot be extensively applied as failing to meet current technical requirements for shortening the line width of the conductive grid in the electrodes, increasing an irradiated area, and improving the efficiency.
The electrodes formed by using said metal paste containing the glass frit result in formation of an area with reduced quality in the metal/semiconductor junction, which is apt to cause recombination of carriers and negatively affect the efficiency of the solar cell.
In light of the above drawbacks, methods for forming electrodes other than the metal paste electrodes containing the glass frit are proposed, such as an inkjet printing method, an electroplating method, or the like. For instance, an electrode layer is formed on a substrate having a P-N diode by conducting the inkjet printing method or the electroplating method, and then an antireflective layer is formed on the substrate to cover the electrode layer. Here, in order to electrically connect the electrode layer to external circuits, openings must be formed on the antireflective layer to expose the underlying electrode layer. Alternatively, an antireflective layer is formed on a substrate having a P-N diode, and then openings are formed on the antireflective layer to expose the underlying silicon substrate. Next, electrodes are formed on the exposed silicon substrate by applying the inkjet printing method or the electroplating method.
Nonetheless, in the aforesaid methods of fabricating the electrodes, the openings of the antireflective layer are formed by way of lithography, laser, or the like, and therefore the entire manufacturing process is complicated. Moreover, the formation of the openings on the antireflective layer for exposing the underlying silicon substrate is likely to cause damage to the silicon substrate.